Researchers photographed J50 last month keeping up with her pod near San Juan Island, Wash. (Katy Foster/NOAA Fisheries)

An ailing killer whale calf that scientists have been trying to treat since August has died, scientists believe.

Ken Balcomb, head of the Center for Whale Research on San Juan Island in Washington state, said research teams declared the three-year-old female, known as J50, dead on Thursday evening.

J50 had not been spotted since last Friday.

Scientists became concerned for the southern resident killer whale in early August, when overhead photography showed she had lost 20 per cent of her body weight. The images triggered a cross-border emergency response and an unprecedented attempt to shoot the whale with doses of antibiotics.